This invention relates to visibly transparent coatings. More particularly it relates to transparent golf ball coatings containing particular ultraviolet light absorbers, sometimes known as "U.V. stabilizers," which selectively absorb short-wave ultraviolet light having a wavelength of less than about 330 nm while permitting the passage of longer wavelengths of ultraviolet light and visible light.
The primary source of ultraviolet light is the sun, which emits a broad spectrum of ultraviolet light as well as other wavelengths of light. The spectrum of sunlight consists mainly of wavelengths greater than about 290 nm, having nearly zero intensity at about 290 nm and a maximum intensity at about 530 nm. The spectrum of sunlight is limited by the filtering effect of the earth's atmosphere. Light having a wavelength in the range below about 400 nm is classified as ultraviolet light, while light in the range from about 400 nm to about 800 nm is classified as visible light.
Ultraviolet light causes photodegradation of certain polymeric compounds. Photodegradation of a polymeric material or its constituents is manifested as discoloration (especially yellowing), brittleness, fading of colored material, and other deterioration. In some instances, ultraviolet light absorbers are added to polymers which are exposed to sunlight. The usual ultraviolet light absorbers function by strongly absorbing the ultraviolet light wavelengths of from about 290 nm or less to about 400 nm, while passing visible light. The energy of the absorbed ultraviolet light is released in the form of infrared energy, which is less energetic and does less damage to the substrate containing the ultraviolet absorber than does ultraviolet light. Absorbance above 400 nm is regarded as undesirable unless it is desired to color or darken the material containing the ultraviolet light absorber.
The presence of ambient ultraviolet light in sunlight can be used to advantage by adding fluorescent materials to a polymer composition. The fluorescent materials of greatest interest here absorb at least some wavelengths of ultraviolet light from sunlight (typically ultraviolet light having a wavelength in the range of about 290 to 400 nm), and emit some of the energy of the absorbed ultraviolet light in the form of visible light. (The residual energy from the incident light is dissipated as heat.) As a result, an unusually great amount of visible light is emitted and reflected from a fluorescent surface, providing a brilliant white or colored surface. Fluorescent materials include optical brighteners, which typically emit bluish light, as well as dyes and pigments, which typically emit blue or other wavelengths of light. Fluorescent materials are commonly adapted to absorb ultraviolet light having a variety of wavelengths and to emit visible light having a characteristic wavelength, and thus a characteristic color.
It would be useful to provide a clear-coated substrate in which the coating and substrate are both protected from short-wave ultraviolet light damage, the substrate (which may be either a bulk material or a base coat) is fluorescent, and the coating is transparent to the long-wave ultraviolet and visible wavelengths of sunlight and of the light emitted and reflected from the substrate through the coating. One combination of this kind would be a clear coating which contains an ultraviolet absorber, so both it and the substrate would be protected from damage due to ultraviolet light, and a fluorescent substrate.
This combination would be desirable, for example, in golf balls. Golf balls must be stabilized to prevent their deterioration in ultraviolet light because they are used (and sometimes left for extended periods) outdoors in the sunlight. At the same time, golf balls are desirably brightened or fluorescent so they are both attractive and easily found on a golf course. This combination would also be useful in other plastics, paints and coating systems for outdoor use, such as automobile finishes, synthetic fabrics, and other articles and materials.
It is undesirable to indiscriminately combine or overlay conventional fluorescent materials with transparent top coats containing conventional ultraviolet absorbers. A typical sunlight-excited fluorescent base material absorbs at least some wavelengths of ultraviolet light in the range of from about 290 to about 400 nm to emit visible light, while typical ultraviolet absorbers have been used to absorb the ultraviolet components of sunlight having the same wavelengths. The fluorescent and ultraviolet blocking materials compete to absorb light of the same wavelengths, so stability is obtained at the expense of useful fluorescence.
The prior art discloses materials containing ultraviolet absorbers and other materials containing fluorescent materials, but does not address the fundamental conflict if both ingredients are used indiscriminately in the same composition, or in a transparent top coat and a substrate for the top coat.